To diversify the function of programmable logic and promote its implementation in, for example, electronic devices, a switch which connects logic cells to each other needs to be downsized to reduce its ON resistance. Switches fabricated using metal precipitation in ion conductive layers which conduct metal ions are known to have smaller sizes and lower ON resistances than other types of semiconductor switches.
Available switches of this type are classified into a two-terminal switch disclosed in PTL 1 and a three-terminal switch disclosed in PTL 2. The two-terminal switch is formed by interposing an ion conductive layer between a first electrode which supplies metal ions and a second electrode which supplies no ions. The two electrodes are switched by metal bridge formation and annihilation in the ion conductive layer. The two-terminal switch has a simple structure and therefore can be fabricated by a simple process with a small element size on the order of nanometers. The three-terminal switch includes integrated second electrodes of two two-terminal switches and therefore ensures high reliability.
As the ion conductive layer, a porous polymer containing silicon, oxygen, and carbon as the main components is desirable. PTL 3 discloses that a porous polymer ion conductive layer can maintain the dielectric breakdown voltage high even upon metal bridge formation and is therefore excellent in operation reliability.
To implement the switching element as a wiring switch for programmable logic, it is necessary to increase the packing density of the switching element by miniaturization and simplify the fabrication process. Since the main wiring material of leading-edge semiconductor devices is copper, a technique for efficiently forming resistive-change elements in copper wiring is desirable.
NPL 1 discloses a technique for integrating a switching element into a semiconductor device using an electrochemical reaction. This technique allows copper wiring on a semiconductor substrate to simultaneously serve as the first electrode of a switching element. With this structure, a process for newly forming a first electrode can be omitted. This obviates the need for a photomask used to form a first electrode, thus limiting the number of photomasks to be added to fabricate a resistive-change element to only two.
In this case, directly forming an ion conductive layer on copper wiring oxidizes the copper wiring surface, leading to a larger leakage current. Therefore, a metal thin film which functions as an oxidized sacrificial layer is sandwiched between the copper wiring and the ion conductive layer. The metal thin film is oxidized by oxygen contained in the ion conductive layer and then partially forms the ion conductive layer. As the second electrode that supplies no ions, platinum or gold that is hard to oxidize or ruthenium that has a given conductivity even after oxidation is used. According to NPL 1, ruthenium suitable for processing is used as the second electrode.
When a switching element which uses a metal bridge and serves as a non-volatile resistive-change element is employed as a wiring switch for programmable logic, holding power is required to hold ON and OFF states for about 10 years without voltage/current application. However, there exists a trade-off between the rewrite current and the holding power in a switching element which uses a metal bridge. For a large write current, the holding power is high because of the formation of a thick metallic bridge; while a thin bridge is formed upon a transition to ON state at a small current for power saving, thus posing a problem involving the holding power of the bridge.
Japanese Patent Application No. 2012-141049 describes a second electrode which contains ruthenium as the main component and is made of an alloy of ruthenium and at least one material selected from titanium, tantalum, aluminum, manganese, zirconium, hafnium, magnesium, cobalt, copper, and zinc. This provides a switching element which achieves both power saving and high reliability.
Japanese Patent Application No. 2013-007349 describes a structure including a bidirectional rectifying element stacked on the second electrode of a three-terminal switch which uses a metal bridge. Since the switching element is kept inactive at the operating voltage of the bidirectional rectifying element or less, switching element selection is easy in a crossbar switch structure forming programmable logic. A general crossbar element structure uses a selection transistor to keep parts other than the selected element inactive, while the use of a built-in rectifying element obviates the need for a selection transistor. This saves the area corresponding to a selection transistor.